1. Field of the Invention
The present invention relates to a positive-working presensitized plate for use in making a lithographic printing plate (hereinafter referred to as a "PS plate") and more specifically to a positive-working PS plate which comprises an aluminum plate carrying an anodized layer and an improved hydrophilic layer applied onto the substrate.
2. Description of Related Art
A lithographic printing plate has conventionally been prepared by imagewise exposing, to light, a PS plate comprising an aluminum substrate provided thereon with a thin layer of a light-sensitive composition and then developing the imagewise exposed PS plate. The aluminum substrate has usually been surface-roughened by, for instance, a mechanical graining such as brush graining or ball graining; electrochemical graining such as electrolytic graining; or a combination thereof to obtain a sprinkled surface, then etched with an aqueous solution of an acid or alkali, anodized and then optionally hydrophilized to give a substrate for lithographic printing plates. A light-sensitive layer is applied onto the surface of the resulting aluminum substrate to give a PS plate. A lithographic printing plate has generally been obtained by imagewise-exposing such a PS plate to light, developing, then correcting and finally gumming up and the resulting printing plate has been fitted to a printing press to perform printing operations.
However, the non-image area of the lithographic printing plate obtained by imagewise exposing a positive-working PS plate and then developing is contaminated due to substances included in the light-sensitive layer which are irreversibly adsorbed onto the non-image area. This results in the formation of so-called remaining color stains which make it difficult to clearly distinguish the image area from the non-image area during the correction process. Further, the correction marks remain on the surface clearly and thus the surface becomes uneven. In the worst case, the correction marks become a cause of background contamination and the resulting printing plate cannot be used in printing.
To solve these problems, there have been proposed various methods, for instance, a treatment comprising immersing an anodized aluminum substrate in an alkali metal silicate solution as disclosed in U.S. Pat. No. 3,181,461; a treatment with fluorozirconate as disclosed in Japanese Examined Patent Publication (hereinafter referred to as "J. P. KOKOKU") No. Sho 36-22063; a hydrophilization treatment with polyvinylphosphonate as disclosed in U.S. Pat. No. 3,276,868; application of an underlying coating of a hydrophilic cellulose containing a water-soluble metal salt as disclosed in U.S. Pat. No. 3,860,426; application of an underlying coating of a sodium arylsulfonate as disclosed in U.K. Patent No. 2,098,627; and application of an underlying coating of polyacrylamide (U.S. Pat. No. 3,511.661). polyvinylphosphonic acid (J. P. KOKOKU No. Sho 46-35685), an amino acid or salt thereof (such as alkali metal salts, e.g., Na and K salts; ammonium salts, hydrochlorides, oxalates, acetates and phosphates) as disclosed in Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 60-149491 (=U.S. Pat. No. 4,801,527), amines having hydroxyl groups and salts thereof (such as hydrochlorides, oxalates and phosphates) as disclosed in J. P. KOKAI No. Sho 60-232998 (=U.S. Pat. No. 4,801,527), or compounds having amino groups and phosphonate residues and salts thereof as disclosed in J. P. KOKAI No. Sho 63-165183. However, these methods are still insufficient in the elimination of the remaining color stains on non-image areas and accordingly the foregoing problem of "background contamination" cannot completely be eliminated. Moreover, these methods make the adhesion between a substrate and a light-sensitive layer insufficient and, therefore, there has still remained room for improvement.